166 



declination, the reference to the meridian is not required. 

 Here, therefore, the theodolite and transit instrument are un- 

 necessary, and the former will be replaced by d^ fixed teles- 

 cope, furnished with a finely divided micrometer scale in its 

 focus. 



A similar apparatus serves for the determination of the 

 changes in the horizontal magnetic force. It is only neces- 

 sary to modify a little the suspension arrangement, and to 

 substitute for the single thread two equidistant threads ; as 

 in the torsion electrometer of Mr. Snow Harris. The needle 

 is then to be turned by the force of torsion, into a position 

 at right angles to the magnetic meridian ; in which position 

 the momentum of the magnetic force is greatest. The 

 changes of position of the bar (read off as before by a fixed 

 telescope with a micrometer scale) will enable the observer 

 to deduce, by an easy formula, the corresponding changes of 

 the magnetic force. Professor Lloyd then entered into some 

 details connected with the theory of the instrument as thus 

 employed ; and he showed in what manner it was to be ad- 

 justed, so tliat a given variation of the magnetic force might 

 produce the greatest variation in the position of the bar. 



A paper was read by Edward S. Clarke, Esq., on an Im- 

 provement which he had lately made in the Sustaining Bat- 

 tery, and on the size proper to be given to the zinc element of 

 sustaining batteries in general. 



The author alluded to the decline of voltaic power which 

 occurs during experiment, and ascribed to M. Becquerel the 

 credit of having assigned its true cause ; referring it, as this 

 philosopher did, to a transfer of the decomposed substances 

 to the respective plates, in such a way as to produce secon- 

 dary currents moving in a direction reverse to the primary 

 current. 



Mr. Clarke also alluded to the fact, that Becquerel was 

 the first person who, to remedy this evil, adopted, in 18^9, 



